EDM power supply for generating self-adaptive discharge pulses

ABSTRACT

An EDM (electric-discharge machining) power supply system for generating self-adaptive discharge pulses wherein an electrode is spacedly juxtaposed with a workpiece across a discharge gap while a dielectric liquid coolant is passed therethrough. The electrode and the workpiece are relatively displaced during the machining of the latter to maintain the gap spacing generally constant via a servomechanism. According to the invention, there is applied across the electrode and the workpiece a direct-current arc-striking voltage sufficient to initiate discharge across the gap while permitting the voltage to build up thereacross to a level constituting a function of conductivity characteristic of the gap and to decay with a discharge across the gap. An analog signal is derived across the gap and represents the voltage buildup and decay thereacross. Machining current flow through the gap across the electrode and the workpiece is triggered by a digital signal derived when the analog signal exceeds a threshold value and initiation of the discharge is induced by the arc-striking voltage. A second digital condition terminates the machining current flow which is controlled by a semiconductive power switch turned on and off instantaneously in dependence upon the digital conditions. A limited current high-voltage source is connected in a closed loop circuit with the electrode, the workpiece and the gap to provide the voltage buildup across the latter, while the voltage across the gap is detected by a voltage divider or the like and the output of this voltage divider is supplied via an integrating circuit in a squaring or gating-type logic device, e.g., a Schmitt trigger capable of producing the digital output for triggering the semiconductive power switch of the machining-current power supply.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of the commonly ownedcopending application Ser. No. 682,824 filed 14 Nov. 1967 and now U.S.Pat. No. 3,539,755 as a continuation-in-part of my earlier applicationSer. No. 493,473 of 6 Oct. 1965 and now U.S. Pat. No. 3,360,683.

GENERAL BACKGROUND

This invention relates to an EDM power supply for generatingself-adaptive discharge pulses and, more particularly, to improvementsin power supply arrangements for the electric discharge machining ofconductive workpieces.

In electrical discharge machining, hereinafter referred to as EDM,conductive workpieces are machined by passing electroerosive powerpulses between a workpiece and a tool electrode spacedly juxtaposedtherewith across an electrode gap flooded with a dielectric coolantwhich also serves to carry away the detritus of the electric dischargemachining process.

In the EDM surfacing or cavity sinking of a conductive workpiece, adiscrete electrical power pulse of a duration of 10¹ 7 to 10¹ 2 secondmay be applied across a relatively carefully dimensioned machining gapwith a spacing, for example, of 0.05-0.005mm., to cause a sparkdischarge or a discharge of the short arc type to momentarily jumpacross the smallest dielectric path between a tool electrode and theworkpiece, constituted as a counterelectrode. The applied electricalenergy is highly concentrated (generally exceeding 10⁵ watts/cm.² with acurrent density of 10⁴ to 10⁹ amp/cm.²) and is localized within thedischarge column, thereby removing particles of that portion of theworkpiece surface upon which the discharge impinges. As a consequence, acrater is formed in the workpiece surface opposite the electrode. Thenext time-spaced pulse may then seek another point of the work surfaceand bridge across the electrode and workpiece a further high-energyelectroerosive discharge. A train of power pulses is thus formed tocreate localized material removal discharges which produce cumulativelyoverlapping craters in the workpiece surface; the total surface is thusmachined uniformly over the parts thereof confronting the electrode andthe machine portion receives a configuration conforming to the shape ofthe electrode.

The latter may be formed with the desired configuration of the cavity orthe shape complementarily desired in the workpiece. During the machiningoperation, small metal or conductive chips or particles are carried awayfrom the gap by the liquid dielectric which floods the gap and isgenerally circulated therethrough, while the tool electrode is advancedrelative to the workpiece by a servo mechanism designed to maintain apredetermined gap spacing or designed to approach the desired gapspacing as accurately as possible.

In high-speed electric discharge machining operations, the energy ofeach individual discharge pulse is generally augmented for a given gapspacing to increase the amount of material removed per pulse; inaddition or alternatively, the pulse repetition rate may be increased byreducing the discharge rest time or interval between one discharge pulseand the next to the minimum conistent with successive pulse formationand a stable cutting condition. In "no wear" operations, in which thetool electrode erosion is limited or eliminated, copper or graphiteelectrodes are customarily employed and are commonly poled positivewhile the workpiece electrode is poled negative, in contrast to normalmachining operations in which the opposite polarity relationship inmaintained. In such "no wear" operation, pulse duration must berelatively long, generally upwards of about 10 microseconds and thepulse amplitude or peak current must be controlled as not to exceed,say, 300 amperes. Excessively long pulses are avoided since they tend toproduce discharges which transform an impulsive short period arc into adamaging thermal arc. Where increased fineness of the surface finish isrequired, a pulse train using narrower pulses is utilized. This lattertype of pulse train results in a substantially reduced rate of removalof the workpiece material and also results in greater erosion of thetool electrode.

PRIOR ART

In commercially available electrical dishcarge machining apparatus,therefore, the pulse generator must be capable of permitting a widerange of selection of pulse parameters in accordance with therequirements of the particular machining operation. In the past, mucheffort has gone into the designing of a versatile and yet efficientperiodic pulse generator. It may be noted in the following discussion ofthe prior art that in substantially every case a pulse generator ofpredetermined frequency is required. For example, in order to overcomethe apparent restriction of relaxation-type pulse generators withrespect to the flexibility of pulse frequency, pulse duration and pulseinterval, a transistorized switching of a DC source with a free-runningmultivibrator has been proposed. The multivibrator is settable toproduce a train of exactly identical signal pulses for a bank ofswitching transistors and ultimately opens and closes the switchingtransistors for a fixed duration and with a fixed interval to connectthe DC machining source at a fixed frequency with the gap independentlyof the conditions prevailing at the latter during the machiningoperation.

It has already been recognized that this type of pulse generator cannotreliably carry out electrical discharge machining because of the factthat the machining gap, as a practical matter, varies with respect tomost of its parameters, i.e., spacing of the electrodes, degree ofcontamination by particles eroded from the workpiece and ionizationproducts of the dielectric, residual ionization, etc. As a consequence,the application of a train of machining pulses of constant amplitude,pulse duration, and interpulse interval results in short circuiting,damaging thermal arcs or discharge failure depending upon the conditionof the gap. To avoid some of these disdavantages, it has been proposedto provide a so-called "isopulse" system permitting current pulses of anexactly equal duration to appear across the machining gap at anundefined frequency with the aid of a pair of monostable multivibrators,one of which is used for fixing the duration of the machining currentpulses while the other establishes the interval between the successivevoltage pulses. Another approach, along similar lines, employs a trainof high-voltage pulses or otherwise imposes high power arc-strikingvoltages at a leading edge of independently generated main voltagepulses, thereby forcing the occurrence of each discharge. In all ofthese systems, the possibility of short-circuiting or thermal arcingremains unsolved or incompletely solved. Indeed, further attempts havebeen made to avoid these difficulties by the cutoff of each pulse independence upon the gap conditions, but here too it is difficult toascertain precisely when a pulse has become transformed into an abnormalpulse and should be cut off.

The problems of how to avoid detrimental short circuiting, thermal arcsand open circuiting (nonfiring) of the gap without decreasing theefficiency of a EDM operation has indeed plagued the art and muchconcern has been expressed about these problems and various solutionsproposed. Thus it has, in general terms, been suggested to determine gapconditions with the aid of a pilot pulse or to otherwise ascertainwhether the gap is in a normal or abnormal state and control themachining pulse accordingly. In one such system, a DC pilot voltageproduces a voltage surge to fire the machining gap, the magnitude of thevoltage surge being measured by a suitable network to determine whetherthe gap is normal or abnormal and, when the former is the case, totrigger a pulse generator to generate the machining pulse of fixedduration. Another arrangement senses the gap deionization after apreceding machining pulse, and in response thereto, operates the pulsegenerator to produce a voltage pulse of preset duration. In all cases,an oscillator-type trigger arrangement is provided to produce pulses ofat least one fixed parameter, i.e., pulse frequency, pulse duration,etc.

It has already been noted that two machining pulses of an identicalduration generally are incapable of providing the identical degree ofelectrical discharge machining or are incapable of ensuring equal energywith respect to the material removed since the working gaps are seldomcompletely identical upon firing. In selecting the machining parameters,the pulse duration is generally determined to attain a desired surfacefinish or the machining rate deviations from this optimum result invariations in surface finish or reduce machining rates. An importantconsideration in EDM systems is that one discharge must be preventedfrom following the same path or striking the same point that wassubjected to the previous discharge; in such cases, a continuous arc mayresult which causes overheating (thermal damage), cracking of theworkpiece and a reduced cutting rate since little erosion takes placeduring thermal arcing. It is therefore important to insure satisfactorydeionization of the machining gap between successive discharges, but toprevent excessive delay between discharges.

APPLICATION SER. NO. 682,824

In my copending application Ser. No. 682,824, mentioned earlier, and ofwhich the present case is a continuation-in-part, I have pointed outthat a pulse generator system for EDM operations must include circuitryfor adjusting the parameters of the pulses. In that system, varying gapconditions are detected to adaptively control machining pulses withoutrespect to the duration, interval and/or repetition rate or meandischarge current within a fixed range whereby problems of shortcircuiting, open-gap pulses and thermal arc are minimized. In the systemof that application there is provided a periodically operated electronicswitch with a preset on-off time having its principle electrodesconnected in series with a direct current machining power supply and thedischarge gap for providing machining pulses to the latter. Sensingmeans is connected to the gap for providing an electrical output signalresponsive to abnormal gap condition while control means is operativelyconnected to the control electrode of the switch for decreasing itson-time and machining pulse on-time responsive to this signal. Delaymeans is coupled between the sensing means and the control means fordelaying the operation of the latter a predetermined time interval afteroccurrence of the abnormal condition. In more specific terms, theelectronic switch has a pair of principal electrodes connected betweenthe power supply and the gap and the pulse generator includes amultivibrator having its output connected to the control electrode ofthe switch. The multivibrator, inturn, is provided with a pair ofelectronic switches biased or coupled for alternate operation and atleast one resistance-capacitance network connected to the controlelectrode of one of the pair of electronic switches (e.g., transistors)for controlling the timing of the alternate operation. In this case, thesensing means may include a variable inductance for providing an outputsignal which is a function of the relatively high frequency dischargesoccurring across the gap during normal cutting, while the delay networkincludes the series connected adjustable inductor and a plurality ofcapacitors coupled in a delay-line configuration with the inductor.

OBJECTS OF THE INVENTION

It is the principal object of the present invention to provide aself-adaptive EDM power supply and a machining method which will obviatethe aforementioned disadvantages and provide improved cutting rates andfinishes, reduce or eliminate the tendency toward short circuiting andnonfiring, and also preclude the formation of thermal arcing.

Another object of this invention is to provide an improved electricaldischarge machining power supply and method which will extend theprinciples set forth in application Ser. No. 682,824, but which will besubstantially nonlimiting with respect to its dependence upon periodicpulse sources.

Still another object of this invention is the provision of an electricaldischarge machining power supply, apparatus and method for optimizingthe discharge machining of metallic workpieces substantiallyindependently of the changing gap conditions generally encountered insuch systems.

SUMMARY OF THE INVENTION

It has now been found that it is possible to optimize or provide selfcontrol for electrically erosive discharge pulses, i.e., pulse"frequency" and the other essential pulse parameters. More specifically,the present invention provides a closed-loop self-adaptive pulsegenerator for optimal timing control (on/off) of an electronic switchingelement or power switch that operatively connects a DC machining-powersource with the machining gap, to provide self-adaptive material-removalspark-producing pulses. The term "closed-loop generator" is used hereinto refer to a system in which the machining gap itself is part of thesignal-pulse generator or the power switch, i.e., the machining gap isconnected in a series circuit with the remainder of the signal-pulsegenerator, so that the gap itself controls the power circuit and theturning on and turning off of the power switch in response to the gapstate, thereby permitting the power to be delivered to the gap in aprecisely regulated discharge and the termination of the discharge byturning off the switch also in response to the gap state. It is thusessential to the present invention that the gap parameters control boththe turning on and turning off of the switch as will become apparenthereinafter. The duration is determined by the sensed parameter, also inrelation to the gap state when the discharge is initiated and/or whilethe discharge is being passed by the gap. Thus, only when the input ofthe system is connected to the gap, is a train of pulses generated whichare considered to be "aperiodic" in the sense that they are formed atundefined repetition rate, or of undefined "off-time" and of undefined"on-time," the discharge time being held preferably within a presetrange. The power switch is desirably of the solid-state type describedin the aforementioned copending application and can carry a high DCpower while insuring rapid and reliable switching operations under heavyload; the "on" and "off" characteristics of such switches can thus begenerally referred to as "instantaneous."

In the apparatus aspect of the invention, a sensor provides themachining gap information manifesting the gap state and this informationis registered in the form of an analog (to the gap variable) by asuitable wave shaper, for example, an integrating network. The level ofthe analog signal changes with time, and represents the gap parametersand variables at a given time and the rate of change closely reflectingthe total gap state as well as the gap state prior to the discharge inquestion. According to a specific feature of this invention, the analogsignal is fed to a threshold gating circuit wherein it is compared witha threshold reference to produce either of two possible digital stateswith sharp interstate transition depending on the analog signal andthereby functions to perform a "squaring" of the analog information inthe from of A/D conversion (analog-digital conversion). Thus, in onestate of the circuit in response to the analog signal, no output appears(corresponding to the digital "zero" signal state), while the circuit inanother state produces a square-wave output corresponding to the digital"one" state. This pair of signals functions as a trigger to close thepower switch and open the latter, thereby causing the switch to bealternately conductive and nonconductive, preferably via an intermediaryamplifying stage.

In accordance with the method aspects of the present invention, anelectrical discharge machining electrode is spacedly juxtaposed to theconductive workpiece across discharge gap while a dielectric liquidcoolant is passed through the gap. The electrode and the workpiece arerelatively displaced during the machining of the latter to maintain thegap spacing generally constant, e.g., via a servomechanism responsive tothe potential or other electrical parameter detected across the gap.Across the electrode and the workpiece, I apply a direct currentarc-striking voltage sufficient to initiate discharge across the gap,while permitting the voltage to build up thereacross to a levelconstituting a function of conductivity characteristic of the gap andalso permitting the voltage to decay with discharge across the gap. Fromthat voltage buildup and decay, I am able to derive an analog signalrepresenting the voltage buildup and decay and to use this analog signalto trigger the machining-current flow across the electrode and theworkpiece upon the analog signal exceeding a first threshold value andupon initiation of discharge by this arc-striking voltage. Using thesame gap-dependent signal, the machining-current flow through the gap isterminated upon the value of the signal, representing the gap voltageattaining a second threshold value, the threshold values beingadjustable in accordance with the desired machining state.

Thus, a first digital condition may be established upon the analogsignal exceeding the first threshold value whereas the logicallyconverse second digital condition is formed upon the analog signalattaining the second threshold value, while the machining-current sourceis advantageously switched at the occurrence of these digital conditionson and off instantaneously.

The arc-striking voltage is applied across the electrode and theworkpiece via a limited current DC source connected across the gap inthe closed-loop or series circuit mentioned earlier, the analog signalbeing derived at least in par while detecting the gap voltage andproducing a signal proportional thereto. Preferably the sensed signal isintegrated to form the analog signal.

Also in accordance with the method aspects of the invention, it may bepointed out that the present system derives an analog signal from thegap indicative of gap recovery from a preceding discharge and related togap conductivity. From the analog signal, there is produced at least onedigital triggering signal deriving an analog signal from the gapindicative of gap recovery from a preceding discharge and related to gapconductivity; the analog signal is used to generate at least one digitaltriggering signal, preferably two such digital triggering signals, uponthe analog signal attaining predetermined threshold values.Aperiodically the source of machining current is turned an and offinstantaneously with the triggering signal.

ADVANTAGES OF THE INVENTION

It has been found that the aforedescribed improved system hassignificant advantages over earlier systems, whether of the nonadaptivetype or of the type having some degree of self-adaptivity. For example,with the present invention it is possible to obtain improved surfacefinishes at a given removal rate or increased removal rates for a givensurface finish; continuous arcing is excluded for all intents andpurposes and hence high quality machine surfaces without burning damagecan be obtained. Moreover, the corners and edges of the workpieces aresharply defined, the regions of the workpieces below the machinedsurface of the workpiece are less affected by heat than in previoussystems, and an extended range of operations is possible in "no wear"modes of operation. Stable cutting conditions are maintained throughoutthe operation and the system is adaptable to full automatic controlwithout operator supervision or intervention.

BRIEF DESCRIPTION OF THE FIGURES

The above and other objects, features and advantages of the presentinvention will become more readily apparent from the followingdescription, reference being made to the accompanying drawing in which:

FIG. 1 is a circuit diagram of an apparatus embodying the presentinvention;

FIG. 1A is a block-type diagram of an EDM machining system illustratingprinciples of the present invention;

FIG. 2 represents waveforms obtaining in the circuit of FIG. 1 atvarious parts thereof;

FIG. 3 is a graph illustrative of the present invention in whichmaterial removal rate is plotted along the ordinate against time alongthe abscissa;

FIG. 4 is a diagram similar to FIG. 1 except provided with a modifiedvoltage and current source arrangement;

FIG. 5 is another power supply diagram for an EDM apparatus according tothe present invention;

FIG. 6 is a series of graphs illustrating the waveforms obtaining inportions of the circuit of FIG. 5;

FIG. 7 is a circuit diagram of the preferred embodiment of the presentinvention;

FIG. 8 is a series of graphs illustrating the waveforms obtaining inthis latter circuit;

FIG. 9 is a circuit diagram of a power supply representing amodification of the system of FIG. 7;

FIG. 10 is a graphic diagram showing the potential (plotted along theordinate) versus time (plotted along the abscissa) of the cycle ofanalog signals under various conditions in the circuit of FIG. 9;

FIG. 11 is a detail view of a modification of the circuit of FIG. 9; and

FIG. 12 is a graph illustrating a practical example using the circuit ofFIG. 9.

SPECIFIC DESCRIPTION I, GENERAL

Referring first to FIG. 1A of the drawing, it may be pointed out that abasic EDM system for the machining of a workpiece W comprises a toolelectrode E formed by the central bore or other aperture arrangementthrough which a liquid dielectric coolant is circulated, e.g., by a pumpp. The receptacle R may be used to recover the detritus-entrainingcoolant which is passed through a filter F adapted to remove from thecoolant particles of metal dislodged by discharge in the apparatus.

The tool electrode E may, in turn, be displaced toward the workpiece Wfor the relative displacement of electrode and workpiece to maintainapproximately the preferred width of the machining gap G.

Also in FIG. 1A, there is shown a system, the circuit for which may bethat illustrated in FIG. 1 and which is designed to show generalprinciples of the invention without obscuring these principles withdetails of circuitry, etc.

Basically, apart from the electrode E, the workpiece W thedielectric-liquid circulation system F, P, R and the servomotor Sm,which is connected with the electrode by a servo drive Sd, there isprovided a gap-responsive circuit, represented by the servo amplifierSa, connected between the electrode E and the workpiece W across themachining gap G. A reference input, represented at Sr, applies a signalrepresenting the desired gap level to the amplifier Sa which acts in themanner of normal servosystems (see my above-mentioned copendingapplication and the earlier applications and patents set forth therein)to produce an error signal or the like controlling the servomotor Sm viaa servocontrol circuit Sc. Hence, during the course of the machiningoperation, the gap G is approximately maintained by appropriate feed ofthe electrode E and the workpiece W via the feedback servosystem shownin FIG. 1A.

In accordance with the principles of my invention, the machining-currentpower source is in part formed by a high-current, relatively low-voltageDC source PS₁, connected in the machining circuit in series with a powerswitch SW across the electrode E and the workpiece W, the latter forminga counterelectrode. The switch SW is electronically controlled andpreferably is a bank of power transistors, e.g., as described inapplication Ser. No. 682,824 or the prior, then pending application Ser.No. 493,473, now U.S. Pat. No. 3,360,683.

This switch has a control electrode SW₁ which is digitally triggered byan one of two digital states into the "on" condition while the seconddigital condition, constituting the logical reversal or opposite of thefirst, triggers the power switch into its "off" state.

In the system illustrated in FIG. 1 an application of the control signale_(o) =0, as illustrated, represents the "off" digital condition whilethe application of the control signal e_(o) =1 represents the "on" stateof the power switch. This electronically controlled switch SW is capableof short or "instantaneous" cutoff and turn-on operation to apply themachining current across the electrode E and the workpiece W.

Apart from the machining-current source PS₁, which is of relatively lowvoltage and relatively high current, I provide a second direct currentsource PS₂ in a gap-responsive self-adaptation circuit which eventuallyprovides the analog output necessary to generate the digital signalwhich operates the power switch SW via a phase (polarity) reversalamplifier Am. The source PS₂ is a relatively high voltage, low-currentsource, e.g., a source in series with a current-limiting impedance 6 asillustrated in FIG. 1, which is connected in a closed-loop or seriescircuit with the gap G across the electrode E and the workpiece W toeffect a voltage buildup thereacross. As has already been noted, thesystem of the present invention omits any pulse-generating system havinga characteristic pulse frequency of any type.

However, to determine gap condition, a sensing device VD is connectedacross the gap G between the workpiece W and the electrode E to providean output signal at its terminal VD₁ which is basically representativeof gap condition. This analog value, preferably voltage, is delivered toan A-D (analog/digital) converter, including, for example, an integratorI_(n) making use of a capacitor across which the sensing signal of thevoltage detector VD is applied. The analog output e_(i) of integratorI_(n) is applied to a threshold discriminator, e.g., unstable devicesuch as a Schmitt trigger Sm designed to switch over when the analoginput attains predetermined threshold values represented as Tv₁, and Tv₂to produce the digital outputs E_(o) =0 when e_(i) <Tv₁, the outpute_(o) =1, when e_(i) >Tv₁ and e_(o) =0 when e_(i) <Tv₂, where Tv₂ <Tv₁.

II. FIRST EMBODIMENT (GENERAL)

The apparatus shown in FIG. 1A, the operation of which will be discussedin detail in connection with FIG. 1, represents a basic system which maybe modified in various respects as described in detail below and evenillustrated in some of the latter figures, for example, a common powersource may be provided in which the gap-responsive adaptation circuit isprovided with impedances limiting the current flow while the machiningcircuit is tapped across the power source to limit the voltage level ofthe machining-current source. Such modifications are described in detailhereinafter.

However, the basic system is clear from FIG. 1A which demonstrates thetotal adaptation of the machining current in the absence of a periodictrigger.

The power source PS₂ serves as an arc-striking voltage source whichbuilds up the potential across the gap and eventually fires the latterto operate the switch SW shortly before or at breakdown of the gap. Thesharp-waveform machining-current pulse thus is triggered across the gapG only upon the breakdown initiated by voltage buildup. However, theduration of the discharge is determined by the decay of the analogvoltage to a predetermined threshold established by the threshold of theSchmitt trigger or some other bistable device as represented, forexample, by the reference input R_(sm) ; it thus is possible to producean aperiodic triggering of pulses whose initiation, duration andinterval all vary from one pulse to the other in accordance with theactually detected gap conditions and represented by the analog signal.

In FIG. 1, I show a self-adaptive power supply circuit for an electricdischarge machining apparatus provided with a servomechanism fordisplacing the electrode relatively to the workpiece and a coolantcirculating system as described in connection with FIG. 1A.

In this system, the electrode 1 is spacedly juxtaposed with theworkpiece 2, constituted as a counterelectrode, across the machining gapG flooded with the dielectric coolant (e.g., kerosene). The machiningcurrent power source comprises the main DC source 3a which is connectedacross the gap G via a digitally triggered power switch constituted by abank of NPN power transistors 4 in series with emitter resistors 4r. Azener diode 5 is connected across the power terminals of the transistor4 to ensure a constant voltage output for the power transistors whichhave their parallel-connected emitter-collector paths in circuit withthe electrode 1, the workpiece 2, the gap G and the machining powersource 3a. A surge-blocking rectifier diode 7 is connected in serieswith source 3a to prevent reverse-current surges or voltages fromdamaging the power source. The bases or transistors 4 are energized incommon by a line 4' to which digital conditions are applied by aclosed-loop, self-adaptive, self-timing system described in detailhereinafter. The main DC source 3a has a low internal impedance and highcurrent capacity so as to be capable, for example, of delivering notless than 50 amperes, the switching transistors 4 being used in suchnumber as to permit the desired peak current to pass through the gapduring machining.

A gap breakdown or "striking" voltage source 3b, constituting anauxiliary DC source with high-voltage and low-current characteristics(e.g., with an output voltage having a no-load value capable ofattaining 100 to 500 volts) is connected in parallel with the mainsource 3a across the electrode 1 and the workpiece 2. In this auxiliarycircuit, there is provided a high ohmic resistor 6 as a current-limitingimpedance to restrict the short-circuiting current from the high voltagesource 3b to a particular maximum level, (e.g., 0.1 to about 1 ampere).It is impossible, therefore, for this auxiliary source alone to sustaina machining discharge. According to the invention and the principles setforth above, the auxiliary source serves to ignite, initiate or strikespark discharge across the gap, alone or in conjunction with a voltagecontribution by the main power source and to explore the gap conditionto obtain the timing of the turn-on of power transistors 4 as will beapparent hereinafter.

Moreover, since breakdown can be initiated across the gap by this highvoltage, low-current auxiliary source, it is possible and desirable toreduce the power rating of the main DC source 3 a to a significantextent, e.g., such that the main power source has an output voltagebetween 10 and 50 volts and just sufficient to maintain an arc-typedischarge, once the same has been initiated by the main source, butincapable of initiating such a discharge alone. The auxiliary powersupply 3b, 6 is thus designed to initiate or "strike" a discharge acrossa deionized gap, while the sustenance of this discharge and the actualmachining operation is provided by the main power source 3b. The overallpower unit can thus be relatively compact, using sources of lower powerrating and distributing the power consumption more economically over theparts of each discharge and upon the power supplies used therefor.

A polarity selector or reversing switch 8 is provided to render theelectrode 6 positive or negative, selectively, with respect to theworkpiece and thus permit the apparatus to operate with reverse polarityor normal polarity as desired for particular machining requirements.

The self-timing adaptation system includes a sensing resistor 9connected across the gap G for drawing information in the form of agap-voltage signal, characterizing the changing conditions at the gap.The output signal is an analog related to the conductivity condition ofthe gap, across which voltage builds up upon full recovery of the gapafter a preceding discharge. The adjustable tap or wiper 9a of theresistor 9 and a fixed terminal 9b develop the integratable signalthereacross and are connected to the integrator network 10 across acapacitor 10b in series with a diode 10a, the capacitor 10 b beingadjustable for calibration, etc. The capacitor 10b charges anddischarges in response to the gap condition variations represented bythe voltage output tapped across terminals 9a and 9b of resistor 9 toproduce the analog output as a function of time with a waverepresentation, e.g., as shown at Vc in FIG. 2.

The circuit 10 may also include a zener diode 10c bridged across thecapacitor 10b in a clipping orientation to clip the analog voltage builtup in the capacitor beyond a predetermined level characteristic, ofcourse, of the rating of the zener diode 10c. Switch 10c' may be closedto tie the zener diode across the output of the integrating circuit. Theintegrating circuit may also include a variable resistor 10d connectedacross the capacitor in a time-constant (R-C) network adjustable toestablish a discharge-time constant for the integrating capacitor 10b.

Connected to the output of the integrator 10, as described generally inconjunction with FIG. 1A, there is an antalog/digital converter in theform of a threshold gating circuit 11, constituted as a Schmitt trigger,energized by the DC source 14 which may be open-circuited via switch 22to render the trigger circuit 11 inoperative. The Schmitt trigger 11establishes the digital conditions which are applied to the powerswitching transistors 4 via the line 4', thereby providing the timingsignal of varying width in dependence upon the incoming analog signalwhich is discriminated with respect to two threshold or referencevalues. The circuit 11 comprises a pair of NPN transistors in conjugatedrelationship wherein an R-C network 11c connects the collector of theinput transistor 11a with the base of the output transistor 11b. Theinput signal is applied from the output side of the integrating circuit10 across the base and an emitter resistor 11r of the input transistor11a, the resistor being adjustable to set a pair of threshold values.The resistors 11d, 11e and 11f are respectively the base-bias resistorof output transistor 10b, collector-bias resistor of input transistor11a and collector-bias resistor of the transistor 11b. Connected to theoutput of the Schmitt trigger 11, there is an amplifying transistor 12of the NPN type which also performs a phase reversal with thephase-reversed output appearing across the output resistor 13. Thebase-bias resistor of the amplifying resistor 12 is shown at 12a.

The Schmitt trigger 11 may be of the type described at pages 389 ff ofPulse, Digital and Switching Waveforms, Millman and TAUB, McGraw-HillBook Co., 1965, and is a bistable circuit having two possible outputstates depending upon the level of the input signal applied at the baseof transistor 11a. The variable resistor 11r when set, establishes apair of threshold values as generally described in connection with FIG.1A so that the circuit acts as a discriminator which converts the levelof the analog signal of the integrating circuit 10 to a digital outputrepresented by the two states of the Schmitt trigger.

When the terminal voltage of capacitor 10b is below a first thresholdvalue, transistor 11a is nonconducting or blocked while transistor 11bis conducting and the amplifying transistor 12 cut off so that no outputsignal appears across the output resistor 13. When, however, theterminal voltage of the capacitor 10b rises above this first thresholdlevel, transistor 11a is biased into its conductive state, transistor11b is cut off and amplifying transistor 12 is switched into itsconducting condition so that a voltage drop appears across the outputresistor 13 and serves to trigger the power switch 4. If, thereafter,the input at the base of transistor 11a falls below a second thresholdvalue, slightly less than the first threshold value, transistor 11b isrendered conducting and amplifying transistor 12 is blocked so that thesignal previously produced, and appearing as a voltage drop acrossresistor 13, is terminated. The positive side of resistor 13 isconnected to the bases (in parallel) of the power transistor 4 which areof the NPN type as previously noted, while the negative side of resistor13 is connected via the resistors 4r to the emitter electrodes of thesetransistors. Thus only when an output signal appears across the resistor13, are the power transistors rendered conducting and held in theirconducting state to connect the main DC source 3a with the machining gapG.

III. FIRST EMBODIMENT (OPERATION)

In FIG. 2, I show the waveforms encountered in the system of FIG. 1 andwherein amplitude is plotted along the ordinate against time as theabscissa; the wave form A represents the terminal voltage V_(c) acrossthe capacitor 10b, waveform B represents the signal voltage V_(o)appearing across the output resistor 13, waveform C represents the gapvoltage V_(gap), and waveform D represents the current pulses Iddelivered to the gap.

Now consider an instant tl at which power transistors 4 are renderednonconductive, sharply terminating a square-shaped discharge pulse P₁with a current of Id and a discharge voltage at a level of Vd. As thegap becomes clear and deionized, voltage Vr (recovery voltage) willbuild up as a result of the auxiliary source 3b of high voltage which isconnected across the no-load gap through resistor 6 and as a result ofthe free capacitance in the circuit.

The sensing resistor 9 continuously monitors the gap voltage and buildsup in capacitor 10b a voltage rise represented by the terminal voltageVc in analog form (waveform A of FIG. 2). As the capacitor voltagebuilds up correspondingly to the gap voltage Vr and exceeds a thresholdlevel Vs of the Schmitt trigger circuit 11, which level is preset tocorrespond to full gap deionization, a transition occurs in the Schmitttrigger circuit 11 whereby transistor 11a is rendered conductive,transistor 11b is blocked and amplifier transistor 12 is renderedconductive, thereby producing a signal voltage V_(o) at the outputresistor 13. The latter signal voltage V_(o), in turn, renders the bankof power transistors 4 conductive (t₂), so that relatively low voltageVm from the main DC source 3a is applied across the gap G in parallelwith the higher recovery voltage Vr.

Thereafter, the recovery voltage Vr and, in response thereto, capacitorvoltage vc, continues to build up until the breakdown of the gap iseffected by the recovery voltage at a magnitude Vb at a time t₃.

The time interval between t₂ and t₃ relates to the gap condition in thisinterval and the magnitude of the gap breakdown or arc-striking voltagevb; hence the peak level of capacitor voltage Vc is generallyproportional to the gap spacing at t₃. Upon the firing of the gap by thearc-striking voltage Vb from the source 3b, discharge current ispermitted to flow from the main source 3a through the power transistors4 which have previously been rendered conductive at time t₂ and theresulting discharge is sustained at the relatively low voltage Vb toproduce the machining pulse P₂.

As soon as the sudden drop of the gap voltage occurs, as consequence ofthe discharge through the gap, capacitor 10b responds to this change viasensing resistor 9, discharging the previously stored charge ofresistors 10b so that its terminal voltage Vc drops linearly (orexponentially) at a rate determined by the capacitors of capacitor 10band the resistance of the draining resistors 10d and 11r thereby formingthe descending flank Vc' of the waveform as represented in the graph ofthe waveforms A in FIG. 2. When this linear or exponential decrease ofthe capacitor terminal voltage Vc reaches and becomes less than thesecond threshold value, represented at Vs', the reverse transitionoccurs in the Schmitt trigger circuit 11 at time t₄ such that transistor11a is cut off, transistor 11b is rendered conductive and transistor 12is switched into its blocking condition, to sharply cut off the signalvoltage V_(o), thereby rendering power transistors 4 nonconductivesubstantially instantaneously. The time t₄ thus corresponds to thetermination of the pulse P₂. One switching cycle "on/off" has thus beenproduced between the times t₂ and t₄ to produce a discharge pulse P₂ ata current level Id and a voltage Vd and with the duration t₃ -t₄adaptively controlled in dependence upon the gap condition.

Since the rate of discharge with time of capacitor 10b is a fixedparameter and the Schmitt-Trigger thresholds Vs and Vs' are also presetparameters, it will be apparent that, in accordance with thisclosed-loop self-timing system, the duration for which the analogvoltage V_(c) is above the threshold level Vs' and, hence the durationof discharge pulse (t₃ -t₄), varies as a function of the peak level ofanalog signal Vc and is a function of the gap breakdown voltage Vdcharacterizing the gap-conductivity condition at the time (t₃) thedischarge pulse commences. The duration T of each discharge pulse can beexpressed by the following formula:

T=t[1-Vs'/Vc(max)]=Bt(1-Vs'/kVb)

where t represents the discharge-time constant for capacitor 10b,Vc(max) represents the peak level of analog signal Vc, Vb is thebreakdown potential of the gap and k is a constant.

At t₄, the gap-recovery voltage Vr again begins to build up providedthere has been no failure of gap deionization At t₅, at which thecapacitor voltage Vc again passed the threshold Vs along the risingflank Vc" of the analog signal, power transistors 4 are again renderedconductive by the sequence discussed above, thereby permitting the mainvoltage Vm to become effective across the open gap. At t₆, the voltagebuildup across the gap fires the latter at a breakdown voltage Vb'higher than that of the preceding cycle (i.e., breakdown voltage Vb).The main voltage from the source 3a instantaneously drops to thedischarge level Vd to produce the steep flank P₃ ' of the next pulse P₃,the capacitor terminal voltage Vc then decreasing at the fixed rate (t)until it again reaches the threshold level Vs' at t₇ (along thedescending flank Vc' ), at which time the power transistors 4 arerendered nonconductive, thereby terminating the discharge pulse P₃ alongthe steep flank P₃ ". The pulse P₃ has a duration t₆ -t₇ and isadaptively controlled in response to the gap voltage and spacing at thetime when the gap was fired. The longer duration of the pulse P₃ mayrepresent slight increase in the gap spacing or some other gap conditionrepresented in a higher resistance of the gap.

Following this cycle, for example, if the gap G is not fully deionizedor is in a condition inadequate to effect a power discharge, the gapvoltage Vr will recover to a slight degree or will fire the gap at arelatively low level, e.g., that represented at Vb", below the fulldeionization level. In such case, only a slight short-circuiting currentfrom auxiliary source 3b (limited by resistor 6) may pass through thegap as represented by the current pulse Id' at time t₈. Thereafter, therecovery voltage Vr again builds up and finally exceeds the deionizationlevel permitting power switches 4 to become conductive at t₉ while thecapacitor terminal voltage Vc follows the recovery voltage Vr. At timet₁₀, a discharge pulse P₄ is initiated by the arc-striking voltage Vb'".Since this striking voltage is relatively low, the discharge pulse P₄ iscaused to terminate with a correspondingly narrow duration t₁₀ -t₁₁.

The subsequent energization of the power switch arrangement 4 iseffected at t₁₂ and is followed by the relatively wide open-gap intervalt₁₂ -t₁₄ as a consequence of an excessive gap spacing, represented forpurposes of illustration. In the condition, the gap does not readilybreak down. Within this interval, the analog signal Vc is shown to reachits maximum level permitted by the breakdown level of the zener diode10c which, if provided, limits an excessive voltage rise acrosscapacitor 10b and thus is effective to avoid an unduly prolonged pulsewhich might cause thermal deterioration of the machined surface. Theprovision of a zener diode, together with an adjustable time constantfor the capacitor 10b permits each individual power pulse to beautomatically optimized within a predetermined fixed range to meet anyparticular machining requirements.

In the preadjustment of the aforedescribed circuit arrangement, a pulseamplitude and a standard pulse duration are determined in considerationof the intended degree of surface finish and also in accordance withwhether or not tool wear is to be permitted. The pulse amplitude is thenset by adjusting the number of switching transistors, e.g., byconnecting the desired number in parallel in the power switch 4. Thestandard pulse duration is established for the most appropriate gapstate which will prevail during the stable machining operation bysuitably adjusting the time constant of the integrating network and/orthe threshold levels Vs and Vs' of the SCHMITT-Trigger Circuit. In themachining operation, it can be assumed that one discharge pulse spansthe shortest dielectric path whose spacing and condition are mostappropriate to the finish and the machining rate desired. The dischargewill then automatically be sustained for this fixed standard duration(e.g., the period t₆ -t₇) and remove the desired amount of materialwhile leaving a crater as expected for the intended result. The nextdischarge is automatically initiated after the lapse of the minimumdeionization period under the existing gap state and when the requisitebreakdown potential is attained. If the breakdown potential isrelatively low as is represented at t₃ or t₁₀, this is an indicationthat the gap is relatively narrow, i.e., the discharge has beengenerated across a relatively narrow shortest dielectric path. Thedischarge duration is automatically determined by this indication, madeby the analog voltage, and the discharge occasioned across this reducedgap is permitted to cease with optimum machining performance, i.e.,without doing excessive work or expending excessive energy which mightlead to gaseous discharge, the formation of thermal arc, etc.

IV FIRST EMBODIMENT (EXAMPLE)

A tungsten carbide workpiece of type G, having a performed hole of 8.6mm., in diameter, was machined by EDM with a silver-tungsten alloy toolelectrode of a diameter of 10 mm., to widen the hole. The liquiddielectric coolant with derosene circulated through the hole in theworkpiece. To machine to a depth of 35 a conventional periodic pulsegenerator equipped with short circuit protection circuitry required 35minutes to yield a machined product with a surface finish of 10 micronsH (max), the initial removal rate of 0.75 g/min., declining as themachining proceeds due to the occurrence of continuous arc discharge atfrequent intervals, to a removal rate as low as 0.1 g/min., at the finalstage. The machining operation by these conventional techniques isrepresented in broken lines in FIG. 3.

With the system of FIG. 1, in accordance with the present invention, thetotal operation required 22.5 minutes and achieved better surface finishwith a substantially constant removal rate of 0.5 g/min throughout themachining operation as represented in solid lines in the graph of FIG.3. Moreover, the power supply of FIG. 1 has been found to be especiallyefficient in the sinking of deep holes, the formation of relatively deepdies and the machining of small areas in which trouble is most oftenencountered in conventional systems.

V. SECOND EMBODIMENT

(General Description and Operation)

In FIG. 4, there is shown a power supply circuit that is generallysimilar to that of FIG. 1 and operates in a corresponding manner exceptthat in this arrangement a single direct current source 103 is providedand serves both as the source of machining current and as a source ofthe auxiliary voltage. In this embodiment, the rectifier 7 may beomitted and the high-ohmic current-limiting resistor 106 connectedbetween the reversing switch 8 and one terminal of the DC source 103which may deliver high voltage and high current. The current drain fromthis source in the auxiliary mode is restricted by resistor 106 whilethe voltage applied during the high-current discharge is regulated bythe zener diode 5. Of course, the number of power-switching transistors4 in series with the source 103 determines the machining current. Withinthe meaning of the terms used in the present disclosure, the auxiliaryvoltage source (high voltage, low current) is represented by the source103 in series with the current-limiting resistor 106, while thelow-voltage, high-current source is represented by the source 103 inseries with the power transistors 4 bridged by the zener diode 5.

VI. THIRD EMBODIMENT (GENERAL)

In FIG. 5, there is shown a modified circuit arrangement in which themachining current source 3a, 7, the power switch arrangement 4, thezener diode 5 and the auxiliary voltage source 3b, 6 are connected to anelectrode arrangement 1, 2 across the gap G via a reversing switch 8 asdescribed in connection with FIG. 1, while the sensing element is avariable resistor 9 bridged across the gap G and providing a voltagedrop across the terminals 9a and 9b which is fed to the analog-signalnetwork.

In this system, between the sensing resistor 9 and the integratingnetwork 10, which is of the type described in connection with FIG. 1 andfeeds the SCHMITT-Trigger 11, either or both of two threshold-leveloperating elements, such as the zener diode 15 and a SCHMITT-Triggercircuit 16, are provided to register upon the integrating network 10only that portion of sensed gap voltage which is discriminated by one orboth of these circuits components.

VII. THIRD EMBODIMENT (OPERATION)

In FIG. 6 there are shown waveforms detected at the tapped resistor 9(waveform A), across the integrating capacitor 10b of the circuit ofFIG. 5 (waveform B), and at the output resistor 13 (waveform C), whilethe corresponding machining pulses, in terms of current, are representedas waveform D.

When the tapped voltage across the gap (Vr) exceeds a threshold level orbreakdown voltage ZV of zener diode 15 (waveform A) as shown at t₁₆, thediode becomes conductive.

The cutoff voltage ZV of the zener diode 15 is set, as shown in FIG. 6,to be slightly higher than the upper threshold value of theSchmitt-trigger circuit 16. Thus, when the voltage Vr causes the diode15 to become conductive, a first transistor 16a of the Schmitt-triggeris rendered conductive to cut off the second transistor 16b as generallydescribed in connection with the Schmitt-trigger 11 with respect towhich Schmitt trigger 16 is similar. The point at which the zener diodebecomes conductive is represented at t₁₆ in FIG. 6. At this point, theintegrating capacitor 10b which has previously been short circuited bythe second transistor 16b, begins to charge along the rising flank Vc'as represented by the waveform B, with the voltage across the capacitorincreasing at a predetermined rate (established by the chargingconstant) until the signal voltage sensed by resistor 9 suddenly dropsbelow the zener level of the diode 15 in response to gap breakdown tatime f₁₈. Prior to this, the capacitor voltage V_(c) crosses the firstthreshold Vs of Schmitt trigger 11 to render transistors 4 conductiveand produce the voltage peak Vp as a contribution by the machiningcurrent source 3a. The switching point is represented at time t₁₇.

Upon the breakdown of the gap at time t₁₈ by the voltage buildup to thepoint Vr' corresponding to the breakdown potential of the gap, themachining pulse P₅ is initiated. Also at this point, capacitor 10b (FIG.5) begins to discharge, thereby producing the descending flank Vc" ofthe analog pulse as described in connection with FIG. 2. When thecapacitor potential falls below the second threshold Vs' of the Schmitttrigger circuit 11 (t₁₉), the machining current pulse 5 is sharplyterminated. After termination of the pulse, recovery commences asrepresented by the next increase in the potential across the gap voltageVr" which at time t₂₀ attains the breakdown level of zener diode 15 toform the next rising flank Vc'" of the analog voltage across thecapacitor 10b, thereby switching the power transistors 4 at t₂₁ andrepeating the cycle. The response of the circuit to failure of gapdeionization, to open-circuiting of the gap, etc., is similar in thesystem described in connection with FIG. 2.

VIII. FOURTH EMBODIMENT (GENERAL)

In FIG. 7, there is shown another closed-loop self-adaptive pulsegenerator arrangement embodying the principles of the present inventionand constituting the preferred system. Here again, the electrodes 1, 2and the gap G are connected in a series circuit with the high-voltagesource 3b and a current-limiting resistor 6 preventing excessive shortcircuit flow through this auxiliary pulse-generating network. Themachining current is supplied by the lower-voltage source 3a in serieswith a reverse-surge-blocking diode 7. Between the machining-currentsource and the electrode arrangement 1, 2, there are provided theswitching transistors 4 in a bank in which the number of transistorsdetermines the applied current. Here, too, a variable resistor 9 spansthe gap G to produce at its terminus 9a, 9b, the signal voltagerepresentative of gap condition.

In this embodiment, however, means is provided whereby control of pulseduration can be effected within about 20 percent of the predeterminedoptimum discharge duration, i.e., ±10 percent. In this arrangement, thetap 9a or resistor 9 is connected to a rectifying diode 10a in serieswith a resistor 10f across the integrating capacitor 10b while aclipping zener diode 10c is provided across the capacitor 10b. Betweenthe capacitor 10b and the positive terminal of the source 14 is provideda resistor 10e the output of the integrating circuit feeding theSCHMITT-trigger circuit 11, the digital output of which is applied viathe amplifying and phase-reversal transistor 12 to the bases of thetransistors 4 via a rectifier 12b.

As in the previous embodiments, therefore, the sensing resistorcontinuously monitors gap variation and registers across its tappedportion 9a, 9b a signal proportional to the changing gap voltage orresistors. The capacitor 10b is here designed to charge and dischargedepending upon the sensed gap signal balanced with an oppositely poledfixed voltage reference source 14 and also operates the threshold-gatingcircuitry 11 and the amplifying transistor 12.

IX. FOURTH EMBODIMENT (OPERATION)

As can be seen from the FIG. 8, waveform A, in which the analog voltageVc derived across the capacitor 10b is represented, upon breakdown ofthe gap accompanied by a sudden drop of the gap signal, capacitorvoltage Vc begins to rise at a time t₂₂ along the steep flank Vc. Inaccordance with an important aspect of this system, Vc rises at avariable rate determined both by its charging time constant (i.e., theproduct of the resistance of resistor 10e and the capacitance of thecapcitor 10b) and the sensed gap signal. The level of capacitor voltagein analog form is discriminated by the Schmitt trigger circuit 11 whosevariable resistor 11r controls the thresholds of the circuit. Thus,while the capacitor voltage VC along the flank Vc' remains below thefirst threshold Vs of the Schmitt trigger, the bank 4 of powertransistors is conductive to sustain the pulse P₆. However, when thisthreshold level is reached (time t₂₃), the gating circuit 11 inverts toterminate pulse P₆ by cutting off the power transistors 4.

Upon termination of the machining pulse P₆, a gap-recovery voltage Vr'builds up across the gap assuming, of course, full deionization of thelatter, as drawn from the auxiliary voltage source 3b, the sensingresistor 9 responding to this buildup as represented by the descendingflank Vc" as shown for waveform A. At the point t₂₄, the descendingflank Vc" traverses the threshold level Vc' and the power transistors 4are rendered conductive so that a further rise in the recovery voltageVr' to its peak at t25 causes breakdown across the gap and the formationof the next discharge pulse P₁ with a time interval between the pulsesas represented at t₂₃ -t₂₅. The terminal voltage at capacitor 10breaches its minimum at t₂₅. The machining cycle then proceeds withadaptively determined pulse duration, pulse spacing and pulse initiationuntil some defect occurs in the sequence.

Also in this system, the duration of each discharge pulse is found tovary adaptively in accordance with the state of the gap upon firingafter the application of the voltage pulse, the state of the gap whilethe discharge is sustained, and the charging time constant of capacitor10b. Thus it is possible to establish a standard pulse duration to meetthe requirements of any particular machining operation by suitablyregulating the capcitance of the integrating capacitor 10b for a givenvalue of the charging resistor 10e or vice versa. Resistor 11r, whichestablishes the threshold level Vs and Vs', may also be set, thesesettings being designed to establish an optimum gap state and spacingwhich may prevail when the gap is fired and while the discharge issustained. It has been found that, when each individual discharge pulseis controlled to vary within a range of ±5 to ±20 percent (preferably±10 percent) of the standard duration in dependence upon the changinggap state, best results are obtained with respect to removal rate,surface finish, accuracy, quality of machined surface, stability ofoperation and minimal wear of the tool electrode (in a "no wear" mode).

If the gap discharge is initiated at a relatively wide spacing and,accordingly, with a higher recovery voltage Vr, this information (to theextent that it is within the permissible deviation indicated above) isreflected in the nature of the discharge. For example, pulse P₈ has alonger duration corresponding to the higher recovery voltage Vr"necessary to break down the gap than is the case for the pulse P₆ or thepulse P₇.

However, if there is an excessive rise of recovery voltage across thegap owing to wider gap spacing, the capacitor terminal voltage Vc isprevented from going excessively negative by the diode 10b which clipsthe negative signal at a level Vd (waveform A of FIG. 8). The clippingeffect is shown between the time intervals t₂₇ and t₂₈. During thedischarge, if the gap voltage is relatively high as shown for theinterval t₂₆ -t₂₈, capacitor terminal voltage Vc rises relatively slowlyto prolong the discharge pulse P₈. If the discharge voltage isrelatively low, indicating sufficient gap deionization and a possiblegas discharge, the pulse is correspondingly narrow, as will be apparentfrom the interval t₄₀ 1/8-t₄₁. At time t₃₃, a premature discharge isshown to take place before complete gap deionization is attained. Inthis case, only the limited current which may be drained at shortcircuit, can be drawn from the auxiliary source 3b. As long as therecovery voltage does not appear, the capacitor terminal voltage Vcremains well above the Schmitt threshold Vs', thereby holding the powertransistors 4 nonconductive. It is also possible to avoid an excessivebuildup of the capacitor terminal voltage within the interval t₃₄ -t₃₅by using the zener diode 10c with a zener level slightly higher than theSchmitt trigger threshold Vs.

X. FIFTH EMBODIMENT

It has been found that, while the aforedescribed embodiments provideexcellent machining performance in deep hole drilling by electricdischarge machining and EDM deep-die sinking and wherever the removal ofthe particles of the workpiece is a problem, the removal rate remainscomparatively slow by contrast with removal rates at the initial stagesof conventional systems as can be seen from the graph of FIG. 3.

It has been found to be possible to increase sharply the initial removalrate by comparison with conventional nonadaptive systems and yetmaintain an improved removal rate throughout the machining operation byaltering the rate of conversion of the gap signal, e.g., thegap-recovery voltage, into the integrated analog signal as machiningproceeds. A system of this type is shown in FIG. 9. Here again theelectrode 1 is juxtaposed with the workpiece 2 across the gap G and isflooded with a dielectric liquid coolant as described in connection withFIG. 1A. Here too, relative displacement of the electrode and theworkpiece to maintain approximately constant gap width, is effected by aservomechanism of the type shown in FIG. 1A and described in connectiontherewith. Also, the main machining current is here supplied by arelatively low voltage machining current source 3a in series with areverse-surge suppressing rectifier diode 7 in series with source 3a, apower switch 4 and the gap G.

The power switch 4 here comprises a cascade arrangement of NPNtransistors in an amplifying circuit of the type illustrated anddescribed in the aforementioned application or U.S. letters patent andincluding a bank of main power transistors 4a, 4b and 4c whoseemitter-collector electrodes are in series with the source 3a and thegap G. A preliminary amplifying arrangement is constituted by a pair oftransistors 4d and 4e whose emitter-collector electrodes lie in serieswith the source 3a and the output resistor 4f across which develops thesignal for triggering the power trnasistors 4a and 4c. The transistors4d and 4e are, in turn, triggered by the application of a signal to thebase of NPN transistor 4g whose emitter is tied in parallel to the basesof transistors 4d and 4e and which has an emitter-collector network inseries with the output resistors 4h and the source 3a.

A resistor 6 is connected in series with the high-voltage auxiliarysource 3b to limit the current drain from this source at shortcircuiting of the gap. Furthermore, the circuit of FIG. 9, which isbasically a modification of that of FIG. 7, includes a voltage-dividerresistor 109 bridged across the electrodes 1, 2 to sense the gapcondition, a diode 10a connected to the output terminal 109a of thisresistor which produces the sensing signal across the terminal 109a,109b. Through the rectifier 10a, the signal may charge a capacitor 10b,or, as illustrated, may be bucked against the charge of the capacitordelivered from source 14 via the variable charging resistor 10e, a zenerdiode 10c being connected across capacitor 10b. The integrated signalcan then be applied to the base of the transistor 11a of a Schmitttrigger 11, the thresholds of which may be set at resistor 11r aspreviously described. Moreover, the Schmitt trigger 11 has an outputtransistor 11b which applies its signals to the amplifying andphase-reversal transistor 12, the latter in turn energizing the powerswitch 4 as previously described.

The sensing resistor 109 is provided with a multiplicity of taps 91, 92,93-9n adapted to be connected in circuit by the selector switch 23 whichhas the terminal 109a to connect one or more of the taps in a voltagedivider arrangement across the gap G. A switch 21 of a timer 20controlling the motor M of the selector switch 23 is ganged with powerswitch 22 in series with the auxiliary source 3b to initiate andterminate a machining operation depending upon whether the switches 21,22 are open or closed. The timer 20 sets the selector switch 23 toconnect the uppermost tap 91 at the start of machining and after apredetermined time interval, shifts the switch 23 to so connect thesuccessively lower taps 92, 93-. The several taps serve to permitcharging of the capacitor at different rates in accordance with theproportionality constant of the voltage divider network associated withthe particular tap. The terminal voltage of capacitor 10b, when the tap91 is in circuit, shows a sharper descending flank (FIG. 10) than thecorresponding capacitor voltage for taps 92 and 93. Hence, the thresholdVs' of the Schmitt trigger is reached more rapidly (time t₄₂) when tap91 is in circuit than for taps 92 and 93 whose threshold time arerepresented at t₄₃ and t₄₄, respectively in FIG. 10. Thus, in the firststage switchover of the Schmitt circuit to render the power transistor 4conductive may take place sooner in early stages and later during thelast stages of machining, thereby increasing the machining rate at thebeginning. In the first stage, for example, the gap voltage may build upto a level of 40 volts before breakdown with increase in potential to50, 60, 70 and 80 volts in the second, third, fourth and last stages ofthe machining operation.

Since in the initial stage of machining, the removal of particles ismore readily accomplished, it has been found that each pulse may betriggered with a relatively lower gap recovery voltage (without thermalarcing) while later stages are effected with higher potentials so thatthorough deionization is assured.

EXAMPLE

In FIG. 12, I show the removal rate in terms of gr./min., plotted alongthe ordinate, against time plotted along the abscissa. The graph relatesto the machining of an iron workpiece using "no wear" operatingtechniques with a copper electrode of 10 mm. in diameter to a surfaceroughness of 35 microns H(max). In this graph, the broken line Xrepresents an adaptive machining system of the type characterizing theprior art while the dot-dash line Y represents the system obtained withthe circuit of FIG. 7 and the solid line graph Z represents the resultsobtained with the circuit of FIG. 9 with voltage steps in the indicatedincrements across the gap.

In FIG. 11 there is shown a modification of the system of FIG. 9 whereina wiper 123 sweeps the resistor 109' continuously rather than in stepsunder the control of a continuous timing drive 120 to delay theattainment of the threshold Vs' with the course of machining. Thissystem is essentially equivalent to that of FIG. 9.

I claim:
 1. A method of machining a conductive workpiece comprising thesteps of:a. spacedly juxtaposing an electrode with said workpiece acrossa discharge gap while passing a dielectric liquid therethrough. b.relatively displacing said electrode and said workpiece during themachining thereof to maintain the gap spacing generally constant; c.applying across said electrode and said workpiece a direct currentarc-striking voltage sufficient to initiate discharge across said gapwhile permitting said voltage to build up thereacross to a levelconstituting a function of conductivity characteristics of said gap andto decay with discharge across said gap; d. deriving an analog signalrepresenting the voltage build up and decay across said gap; and e.triggering a machining current flow through said gap across saidelectrode and said workpiece upon said signal exceeding a firstthreshold value and upon initiation of a discharge by said voltage, andterminating said machining current flow through said gap upon detectionof a value of the analog signal declining to a second threshold valueless than said first threshold value.
 2. The method defined in claim 1,further comprising the steps of establishing a first digital conditionupon said signal exceeding said first threshold value and a logicallyconverse second digital condition upon said signal attaining said secondthreshold value; and switching a machining current source connectedacross said electrode and said workpiece on and off instantaneously inresponse to the establishment of said first and second conditions,respectively.
 3. The method defined in claim 2 wherein said arc-strikingvoltage is applied across said electrode and said workpiece by alimited-current DC source connected across said gap in a closed-loopcircuit therewith, said analog signal being derived by detecting the gapvoltage and producing a sensed signal proportional thereto, andintegrating said sensed signal to form said analog signal.
 4. In theelectric discharge machining of a conductive workpiece wherein anelectric discharge machining electrode is spacedly juxtaposed with theworkpiece across a machining gap flooded with a dielectric liquidcoolant and a source of electric machining current is connectable acrosssaid electrode and said workpiece to pass an impulsive erosive electricdischarge across said gap, the improvement which comprises the stepsof:a. connecting said electrode, said gap and said workpiece in aclosed-loop circuit with a limited-current voltage source; b. derivingan analog signal from said gap indicative of gap recovery from apreceding discharge and related to gap conductivity; c. producing withsaid analog signal at least one digital triggering signal upon saidanalog signal attaining a predetermined threshold value; and d. aperiodically triggering said source of machining current on and offinstantaneously with said triggering signal, the analog signal beingcompared with at least one threshold value to produce a first digitalstate and with another threshold value less than the said one thresholdvalue to produce a second digital state, one of said digital statescorresponding to said digital triggering signal, said source ofmachining current being turned on and off in accordance with saiddigital states.
 5. The improvement defined in claim 4 wherein saidanalog signal is derived in step b, by the steps of deriving a signalrepresenting voltage buildup across said gap, and integrating thevoltage-buildup signal by controlling the charge and discharge of acapacitor therewith to produce said analog signal.
 6. The improvementdefined in claim 5, further comprising the step of clipping said analogsignal beyond a predetermined level thereof to prevent an excessiveduration of triggering of said source of machining current.
 7. A powersupply for electrical discharge machining wherein a tool electrode isspacedly juxtaposed with a workpiece constituting a counterelectrodeacross a machining gap flooded with a dielectric liquid coolant, saidpower supply comprising:a main source of machining current including anelectronically triggerable power switch connected in series with saidelectrodes and said gap; an auxiliary source of arc-striking voltageconnected in a closed-loop arrangement with said gap for building up avoltage thereacross; sensing means including a voltage-dividing resistorhaving a pair of output terminals connected across said gap forproducing an output indicative of the breakdown conditions of said gap;means including an integrating network having a capacitor bridged acrosssaid output terminals for forming an analog signal from said outputrelated to the gap conditions; threshold gating means including aSchmitt trigger responsive to said analog signal for establishing afirst digital state and a second digital state, said threshold gatingcircuit being connected to said power switch for substantiallyinstantaneously rendering same conductive and terminating conductivityof said power switch upon the occurrence of said digital statesrespectively, said Schmitt trigger having an input transistor with abase connected to said capacitor.
 8. The power supply defined in claim 7wherein said power switch is a bank of power transistors connected inparallel between said main source and said electrodes and having basestriggerable in parallel by said threshold gating means.
 9. A powersupply for electrical discharge machining wherein a tool electrode isspacedly juxtaposed with a workpiece constituting a counterelectrodeacross a machining gap flooded with a dielectric liquid coolant, saidpower supply comprising:a main source of machining current including anelectronically triggerable power switch connected in series with saidelectrodes and said gap; an auxiliary source of arc-striking voltageconnected in a closed-loop arrangement with said gap for building up avoltage thereacross; sensing means including a voltage-dividing resistorhaving a pair of output terminals connected across said gap forproducing an output indicative of the breakdown conditions of said gap;means including an integrating network having a capacitor bridged acrosssaid output terminals for forming an analog signal from said outputrelated to the gap conditions; and threshold gating means including aSchmitt trigger responsive to said analog signal for establishing afirst digital state and a second digital state, said threshold gatingcircuit being connected to said power switch for substantiallyinstantaneously rendering same conductive and terminating conductivityof said power switch upon the occurrence of said digital statesrespectively, said Schmitt trigger having an input transistor with abase connected to said capacitor, and means establishing two thresholdvalues for said Schmitt trigger effective to produce an output signalupon the voltage across said capacitor exceeding one of said thresholdvalues and terminating said output signal upon the voltage across saidcapacitor attaining a second threshold value less than the firstthreshold value, the output signal of said Schmitt trigger being appliedto said power switch for rendering same conductive.
 10. The power supplydefined in claim 9, further comprising amplifier means between saidSchmitt trigger and said power switch.
 11. The power supply defined inclaim 9, further comprising an electronic breakdown device connectedacross said capacitor for clipping the voltage appearing thereacrossupon the latter voltage exceeding a predetermined magnitude.
 12. Thepower supply defined in claim 11 wherein said breakdown device is aZener diode.
 13. The power supply defined in claim 9, further comprisingmeans including a source of direct current and a resistor in series withsaid capacitor and said source of direct current for charging saidcapacitor, said voltage divider being connected to said capacitor so asto buck the charging voltage supplied thereto through said resistor. 14.The power supply defined in claim 9, further comprising a rectifierdiode connected between one of said output terminals and said capacitor.15. The power supply defined in claim 9 wherein said main source ofmachining current includes a direct current source and a bank of powertransistors in series with said direct current source and saidelectrode, and wherein said auxiliary source includes said directcurrent and a current-limiting resistor in series therewith connectedacross said electrodes.
 16. The power supply defined in claim 9 whereinsaid major source includes a relatively low-voltage high-current DCsource connected in series with a bank of power transistors across saidelectrodes and wherein said auxiliary source includes a relativelyhigh-voltage low current DC source connected across said electrodes. 17.The power supply defined in claim 16, further comprising a rectifierdiode connected in series with said low voltage high current DC sourceto block high voltage from said high-voltage low current DC source. 18.The power supply defined in claim 16, further comprising acurrent-limiting resistor in series with said high-voltage DC source.19. The power supply defined in claim 9 further comprising means foradjusting the charging and discharging rate of said capacitor during theelectrical discharge machining process.
 20. The power supply defined inclaim 19 wherein the last-mentioned means includes a plurality of tapsof said voltage divider and sequence-switching means successivelyconnecting said capacitor with said taps.
 21. The power supply definedin claim 19 wherein said voltage divider is a variable resistor, thelast-mentioned means including a wiper adapted to sweep along saidvariable resistor.
 22. A power supply for electrical discharge machiningwherein a tool electrode is spacedly juxtaposed with a workpiececonstituting a counterelectrode across a machining gap flooded with adielectric liquid coolant, said power supply comprising:a main source ofmachining current including an electronically triggerable power switchconnected in series with said electrodes and said gap; an auxiliarysource of arc-striking voltage connected in a closed-loop arrangementwith said gap for building up a voltage thereacross; sensing meansconnected across said gap for producing an output indicative of thebreakdown conditions of said gap; means forming an analog signal fromsaid output related to the gap conditions; threshold gating meansresponsive to said analog signal for establishing a first digital stateand a second digital state, said threshold gating circuit beingconnected to said power switch for substantially instantaneouslyrendering same conductive and terminating conductivity of said powerswitch upon the occurrence of said digital states respectively, saidmeans forming said analog signal from said output including anintegrating network connected between said sensing means and saidthreshold gating means, said sensing means including a voltage dividerconnected between said electrodes across said gap and having outputterminals connected to said integrating network, said integratingnetwork including at least one capacitor in circuit with said outputterminals and chargeable at a rate controlled by a potential appearingacross said output terminals, said threshold gating means including aSchmitt trigger circuit having an input connected to said capacitor andmeans establishing two threshold values for said Schmitt trigger circuiteffective to produce an output signal upon the voltage across saidcapacitor exceeding one of said threshold values and terminating saidoutput signal upon the voltage across said capacitor attaining thesecond threshold value, the output signal of said Schmitt triggercircuit being applied to said power switch for rendering the sameconductive; and a timer for connecting sequentially successive portionsof said voltage divider in circuit with said capacitor for adjusting thecharging and discharging rate of said capacitor during the electricaldischarge machining process.
 23. The power supply defined in claim 22,further comprising a plurality of taps of said voltage divider andsequence-switching means including said timer successively connectingsaid capacitor with said taps.
 24. The power supply defined in claim 22,further comprising amplifier means between said Schmitt trigger circuitand said power switch.
 25. The power supply defined in claim 22, furthercomprising an electronic breakdown device connected across saidcapacitor for clipping the voltage appearing thereacross upon the lattervoltage exceeding a predetermined magnitude.
 26. The power supplydefined in claim 25 wherein said breakdown device is a Zener diode. 27.The power supply defined in claim 22, further comprising means includinga source of direct current and a resistor in series with said capacitorand said source of direct current for charging said capacitor, saidvoltage divider being connected to said capacitor so as to buck thecharging voltage supplied thereto through said resistor.
 28. The powersupply defined in claim 22, further comprising a rectifier diodeconnected between one of said output terminals and said capacitor. 29.The power supply defined in claim 22 wherein said main source ofmachining current includes a direct current source and a bank of powertransistors in series with said direct current source and saidelectrodes, and wherein said auxiliary source includes said directcurrent source and a current-limiting resistor in series therewithconnected across said electrodes.
 30. The power supply defined in claim22 wherein said major source includes a relatively low voltagehigh-current DC source connected in series with a bank of powertransistors across said electrodes and wherein said auxiliary sourceincludes a relatively high-voltage low-current DC source connectedacross said electrodes.
 31. The power supply defined in claim 30,further comprising a rectifier diode connected in series with saidlow-voltage high-current source to block high voltage from saidlow-current DC source.
 32. The power supply defined in claim 30, furthercomprising a current-limiting resistor in series with said high-voltageDC source.
 33. A power supply for electrical discharge machininingwherein a tool electrode is spacedly juxtaposed with a workpiececonstituting a counterelectrode across a machining gap flooded with adielectric liquid coolant, said power supply comprising:a main source ofmachining current including an electronically triggerable power switchconnected in series with said electrodes and said gap; an auxiliarysource of arc-striking voltage connected in a closed-loop arrangementwith said gap for building up a voltage thereacross; sensing meansconnected across said gap for producing an output indicative of thebreakdown conditions of said gap; means forming an analog signal fromsaid output related to the gap conditions; and threshold gating meansresponsive to said analog signal for establishing a first digital stateand a second digital state, said threshold gating circuit beingconnected to said power switch for substantially instantaneouslyrendering same conductive and terminating conductivity of said powerswitch upon the occurrence of said digital states respectively, saidmeans forming said analog signal from said output including anintegrating network connected between said sensing means and saidthreshold gating means, said sensing means including a voltage dividerconnected between said electrodes across said gap and having outputterminals connected to said integrating network, said integratingnetwork including at least one capacitor in circuit with said outputterminals and chargeable at a rate controlled by a potential appearingacross said output terminals, said threshold gating means including aSchmitt trigger circuit having an input connected to said capacitor andmeans establishing two threshold values for said Schmitt trigger circuiteffective to produce an output signal upon the voltage across saidcapacitor exceeding one of said threshold values and terminating saidoutput signal upon the voltage across said capacitor attaining thesecond threshold values, the output signal of said Schmitt triggercircuit being applied to said power switch for rendering sameconductive, said Schmitt trigger circuit including a first transistorconnected to said capacitor and a second transistor connected to saidfirst transistor and producing said output signal of the Schmitt triggercircuit, said means establishing said threshold values including atleast one variable resistor connected in circuit with at least one ofsaid transistors, said capacitor being a variable condenser.
 34. Thepower supply defined in claim 33 wherein said power switch is a bank ofpower transistors connected in parallel between said main source andsaid electrodes and having bases triggerable in parallel by saidthreshold gating means.
 35. The power supply defined in claim 33,further ocmprising amplifier means between said Schmitt trigger circuitand said power switch.
 36. The power supply defined in claim 33, furthercomprising an electronic breakdown device connected across saidcapacitor for clipping the voltage appearing thereacross upon the lattervoltage exceeding a predetermined magnitude.
 37. The power supplydefined in claim 36 wherein said breakdown device is a Zener diode; 38.The power supply defined in claim 33, further comprising means includinga source of direct current and a resistor in series with said capacitorand said source of direct current for charging said capacitor, saidvoltage divider being connected to said capacitor so as to buck thecharging voltage supplied thereto through said resistor.
 39. The powersupply defined in claim 33, further comprising a rectifier diodeconnected between one of said output terminals and said capacitor. 40.The power supply defined in claim 33 wherein said main source ofmachining current includes a direct current source and a bank of powertransistors in series with said direct current source and saidelectrodes, and wherein said auxiliary source includes said directcurrent source and a current-limiting resistor in series therewithconnected across said electrodes.
 41. The power supply defined in claim33 wherein said major source includes a relatively low-voltagehigh-current DC source connected in series with a bank of powertransistors across said electrodes and wherein said auxiliary sourceincludes a relatively high-voltage low-current DC source connectedacross said electrodes.
 42. The power supply defined in claim 41,further comprising a rectifier diode connected in series with saidlow-voltage high-current source to block high voltage from said lowcurrent DC source.
 43. The power supply defined in claim 41, furthercomprising a current-limiting resistor in series with said high-voltageDC source.
 44. The power supply defined in claim 33, further comprisingmeans for adjusting the charging and discharging rate of said capacitorduring the electrical discharge machining process.
 45. The power supplydefined in claim 44 wherein the last-mentioned means includes aplurality of taps of said voltage divider and sequence-switching meanssuccessively connecting said capacitor with said taps.
 46. The powersupply defined in claim 44 wherein said voltage divider is a variableresistor, the last-mentioned means including a wiper adapted to sweepalong said variable resistor.
 47. The power supply defined in claim 44wherein the last-mentioned means includes a timer for connectingsequentially successive portions of said voltage divider in circuit withsaid capacitor. .Iadd.
 48. A process for shaping a workpiece usingelectric current comprising the steps of:positioning a workpieceadjacent a working electrode such that the electrode and the workpieceare separated by a working gap, impressing an intermittent pulse voltageacross said working gap, detecting the time period between the instantat which a voltage is impressed across said working gap and the instantat which a substantial current begins to flow within said working gap;and, controlling the length of time during which said substantialcurrent flows directly in response to the length of said detected timeperiod. .Iaddend..Iadd.
 49. A process as in claim 48, wherein said stepof controlling includes the steps of: reducing the length of time duringwhich said substantial current flows in said working gap below a maximumpredetermined length of time provided said time period is less than apredetermined interval; and, permitting said substantial current to flowfor said maximum predetermined length of time provided said time periodis greater than said predetermined interval. .Iaddend. .Iadd.
 50. Anapparatus for shaping a workpiece by electric current comprising:switching means for impressing a voltage across a working gap between anelectrode and a workpiece, said voltage remaining at a no-load leveluntil a substantial current flows in said working gap, timing means formeasuring the time period during which said no-load voltage existsacross said working gap; and, control means coupled to said no-loadvoltage time measuring means for controlling the duration of flow ofsaid substantial current directly in response to the output from saidno-load voltage time measuring means. .Iaddend.